Field of the Invention
Embodiments of the invention relate to an optical sheet, and more particularly, to an optical sheet which improves luminance and a backlight unit and display device comprising the same.
Discussion of the Related Art
In recent years, the field of displays for visually expressing various kinds of electric signal information is developing rapidly. In line with this, a variety of flat panel displays (FPDs) having excellent characteristics such as thin profile, lightness in weight, and low power consumption have been introduced and are rapidly replacing conventional cathode ray tubes (CRTs).
Examples of flat panel displays include a liquid crystal display (LCD), a plasma display panel (PDP), a field emission display (FED), and an electroluminescence display (ELD). The liquid crystal display is most actively used in the field of display screens for laptops, monitors, and TVs because of its high contrast ratio and excellent display characteristics of a moving picture.
The liquid crystal display may be generally classified as a light receiving display. The liquid crystal display comprises a liquid crystal display panel displaying an image and a backlight unit that is positioned under the liquid crystal display panel to provide the liquid crystal display panel with light. The backlight unit may comprise a light source, a light guide plate for propagating the light provided from the light source towards the display panel, and an optical sheet. Here, the optical sheet may comprise a diffusion sheet for diffusing and focusing light and a prism sheet. The light emitted from the light source in the conventional backlight unit having the above configuration may be focused by the prism sheet.
FIG. 1 is a view schematically showing the focusing of light incident on a prism sheet in a backlight unit of a liquid crystal display according to a related art. Referring to FIG. 1, when light is incident on a prism sheet made up of a base film 10 and a plurality of prisms 20, the light travels, and is refracted at a certain angle due to a refractive index difference between an air layer and the incidence plane of the prism 20. The prisms 20 have an angle θ2 of the incidence surface. The light touching the reflective surface is reflected upward or substantially vertically from the prism sheet and contributes to luminance. In contrast, the light not touching the reflective surface is directed toward a side of the prism sheet and causes light loss. Accordingly, the luminance of the light provided from the backlight unit is lowered, and therefore the overall luminance of the display device may be lowered.